Comparing models to Pliocene climate, when CO2 was last this high

Warming in Arctic may have been greater than our models are showing.

One thing climate modelers like to do is simulate interesting periods of Earth’s climate history, both to study what it was like then and to see how well the model matches real-world climate records. One of those interesting periods is the latter part of the Pliocene epoch, around three million years ago, when the concentration of CO2 in the atmosphere was last as high as it is today.

In a way, the Pliocene gives us the results of the experiment we’re (unintentionally) running today. That is, what does the Earth look like with around 400 parts per million of CO2 in the atmosphere? Rather than showing us how much warmer the planet will be in three or four decades, it’s an indication of where the Earth will settle after the long-term equilibrium between CO2 and the sluggish climate system plays out.

The planet was 2 to 3°C warmer, on average, and sea level was perhaps as much as 30 meters higher at times. The West Antarctic Ice Sheet collapsed during the warmer parts of the Pliocene, submerging the low-lying part of the continent that the ice sits on.

A new study in Nature Climate Change compares simulations from a number of climate models to records of temperature and ecosystem types in the Pliocene. To help us learn from the Pliocene, researchers want their models to be able to reproduce it. Mismatches between model simulations and records of past climate can mean one of three things. Obviously, the simulation can be incorrect for a myriad of reasons. But the record, too, can be off—in terms of temperature or timing. Finally, it can be a combination of the two—and it’s not always obvious which of these scenarios applies to a given result.

In this case, the records came mainly from analyzing changes in plant species found in sediment cores. Understanding the temperature ranges of those species allows researchers to calculate the average temperature and describe the type of ecosystem that dominated in an area, whether grassland or boreal forest.

While the global averages of the model simulations used were all in the Pliocene ballpark, they underestimated (to varying degrees) the warmth near the Arctic. Compared to records in Siberia, for example, the mismatch rose to 10°C or more. That could mean that Arctic amplification—the feedbacks which make the poles, and particularly the Arctic, warm much more than the tropics—was greater in the Pliocene than the models are simulating.

Raising the concentration of CO2 to the high end of Pliocene estimates, or fiddling with the phase of the orbital cycles that determine how sunlight reaches the Earth, enabled a closer match with Arctic temperatures. But that came at the expense of a worse fit elsewhere.

The researchers point to uncertainty in the climate records, which confounds their ability to determine the cause of the mismatch between the models and the records. Using records representing slightly different points in time can create a misleading picture of global temperature patterns by mixing data from warmer and cooler periods. The researchers write, “We conclude that to allow a more robust evaluation of the ability of present climate models to predict warm climates, future Pliocene data-model comparison studies should focus on orbitally defined time slices.”

This study is more about providing fodder for the climate modeling community than uncovering indications of where our warming climate could be headed. Still, it’s important to understand how the Arctic responds to warming since that has such a strong impact on global climate. The models that came closest to the Arctic records used were those with higher overall sensitivities to CO2-induced warming. If that holds true in comparisons with more tightly constrained records, it could put a tick in the column for those higher sensitivities being representative of future climate change.